Pyrosequencing is a method based on the detection of the pyrophosphate group that is generated when a nucleotide is incorporated in a DNA polymerase reaction [1]. Each of the four deoxynucleotides (dNTPs) is added sequentially to the DNA template to be sequenced with a cocktail of enzymes and substrates in addition to the usual polymerase reaction components. If the added nucleotide is complementary with the first available base on the template, the nucleotide will be incorporated and a pyrophosphate will be released. The released pyrophosphate is converted to ATP by sulfurylase, and this ATP is the substrate for a luciferase, e.g. firefly luciferase, which reaction produces visible light. If the added nucleotide is not incorporated, no light will be produced and the nucleotide will simply be degraded by the enzyme apyrase. This pyrosequencing technique, schematized in FIG. 1, has been applied to single nucleotide polymorphism (SNP) detection and other applications [2].
There are, however, inherent difficulties in the traditional pyrosequencing method for determining the number of incorporated nucleotides in homopolymeric regions (e.g. a string of several T's in a row) of the template. Moreover, dATP greatly interferes with the luciferase detection system, which is deficient in the detection of dATP.